This invention relates to electrophotographic marking devices and systems. In particular, the present invention relates to methods and apparatus for matching the output of one marking device with that of another marking device.
A typical marking device, such as a printer, is used to produce typeset-quality output from a client. The client, or computer front end, generally includes an application program that is used to create or find an image to be output. The application program sends the image to the device driver, which serves as an interface between the client and the marking device. The device driver then encodes the image in a page description language (“PDL”), and sends the encoded image to the marking device.
Many conventional marking devices include a raster image processor and a marking engine. The raster image processor receives the PDL-encoded image from the client marking device driver and converts the image into a raster, or a grid of lines. Each line includes a series of pixels that may be turned on or off as necessary to represent the contents of the image. The raster image processor sends this rasterized stream of pixels to the marking engine for output to a receiver.
Conventional marking processes are limited in that they cannot produce a continuous spectrum of tone levels within an image. Instead, modulated patterns are used to simulate different tone levels. One common form of modulation is the use of halftones. For instance, many typical black and white printers use halftones to produce a variety of gray levels, such as 30% white or 60% white. The use of halftones involves dividing the rasterized image into halftone cells, each of which contains a fixed number of pixels. A continuous tone spectrum thus may be approximated by turning on a certain percentage of the pixels in each halftone cell. Theoretically, a 50% gray level may be achieved by turning on 50% of the pixels in a halftone cell on a black and white printer. In practice, the actual gray levels produced by different marking devices vary somewhat from this ideal. Three factors that typically affect how a pixel is produced by a given marking device include screen angle, spot function, and frequency (i.e. lines per inch). Variation in these factors often causes some marking devices to produce output that is too dark while other marking devices produce output that is too light. Accordingly, a 50% halftone produced by a particular marking engine may result in a gray level that is slightly more or less than 50% white.
Slight variation from the ideal standard spectrum of gray levels may be tolerable in many circumstances. However, there are situations in which the variation between outputs from different marking devices presents a problem. For instance, a single print job may be produced in parts, with different parts being produced on different printers. In this case, it may be desirable to match the outputs of the different printers to each other or to an objective standard, thus ensuring consistency throughout the entire print job output.
Methods of calibrating a marking device exist to compensate for variation from the ideal standard spectrum of tone levels. Existing methods typically involve calibrating the tone transfer function of a marking device to match an ideal objective standard. The tone transfer function is the relationship between a tone level specified by the client application and the resulting halftone level, which is the percentage of pixels that are turned on in a halftone cell. This function can be adjusted to offset any variation from the ideal spectrum of output tone levels. In this way, the marking device can approximate an ideal one-to-one standard relationship between a client-requested tone level and the resulting output tone level.
Calibration of a marking device is typically performed using a densitometer. A densitometer is an instrument that measures the “density” of an output tone level. For instance, the density of a gray level output from a black and white printer is the percentage of area that is black. A higher percentage of black area results in a higher density and a darker overall gray level. Densitometers can be used to measure the density of various output tone levels and detect any variations from the ideal standard. Alternatively, sample output tone levels can be compared to templates containing standard patterns that are known to accurately match a set of ideal tone levels. In each of these cases, disparities between the output and the ideal spectrum of tone levels can be identified. Once these disparities are identified, appropriate adjustments can be made to the tone transfer function to calibrate the marking device to the ideal standard. These methods are known in the art.
Known calibration methods, however, require some objective measure of the ideal spectrum of tone levels. For instance, special equipment such as a densitometer may be required to determine the true density of a sample output tone level. Moreover, a skilled operator usually is desired to accurately assess the results obtained from the densitometer. Yet a densitometer and a skilled operator are not always available. Alternatively, a template with accurate standard patterns is required to provide some objective measure of the ideal tone spectrum. However, a standard pattern also may not be available. Indeed, in many circumstances, it is simply not practical to match the output of a marking device to the ideal standard spectrum.
When it is desirable to produce consistent output among two or more marking devices, however, it may not be necessary to achieve the objective ideal standard with each of the devices. For instance, it would often be sufficient to match the output of a first printer with the output of a second printer, regardless of whether the two outputs match the ideal standard. It is, therefore, an object of the invention to provide improved methods and systems for matching the outputs of two or more marking devices without the need to reference an objective tone level standard. It is also an object of the invention to provide improved methods and systems for matching the outputs of two or more marking devices without the use of additional equipment, operators, or standard patterns required to achieve an objective standard tone spectrum.